Implement towncrier

[RuneDominik]

Copy over towncrier set-up from ctapipe and add to CI. I've created some news-fragments for older PRs for testing reasons.

[codecov[bot]]

Codecov Report

Base: 91.04% // Head: 91.05% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (dc1d39f) compared to base (2629bdc). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #207 +/- ## ========================================== + Coverage 91.04% 91.05% +0.01% ========================================== Files 40 41 +1 Lines 1697 1699 +2 ========================================== + Hits 1545 1547 +2 Misses 152 152 ``` | [Impacted Files](https://codecov.io/gh/cta-observatory/pyirf/pull/207?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cta-observatory) | Coverage Δ | | |---|---|---| | [pyirf/\_\_init\_\_.py](https://codecov.io/gh/cta-observatory/pyirf/pull/207?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cta-observatory#diff-cHlpcmYvX19pbml0X18ucHk=) | `100.00% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cta-observatory). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cta-observatory)

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[maxnoe]

Nice, thanks!